1. Field of the Invention
The present invention relates to an idle speed control device for control engine rotation speed to a target value in an idling state, and particularly to an idle speed control device where feedback control is selected in an idling state, and open loop control is selected in a non-idling state.
2. Description of Background Art
An idle speed control device provided with a bypass passage for bypassing a throttle valve connected to an intake pipe and an idling control valve (idle air control valve IACV) for controlling opening amount of the bypass passage, for adjusting the IACV based on engine parameters such as engine cooling water temperature and engine rotation speed, in order to control engine rotation speed to a target value in an idling state, is disclosed in Japanese patent application No. 2000-45835, and Japanese patent application No. 2002-106387.
In Japanese patent application No. 2002-106387, in order to control IACV opening amount to an appropriate value, feedback control is adopted in an idling state and open loop control is adopted in a non-idling state. Also, when shifting from an idling state to a non-idling state, in order to relieve a phenomenon of engine blowing caused by sudden reduction in intake air amount, as shown in FIG. 6, technology to cause a gradual reduction in intake air amount is disclosed.
The fact that there has been a shift from an idling state to a non-idling state, as also shown in FIG. 6, is often determined based on whether or not throttle opening amount θ th has exceeded a reference opening amount θ ref, and this reference opening amount θ ref is set to an extremely low opening amount. Accordingly, immediately after shifting from an idling state to a non-idling state, intake air amount supplied through the throttle valve remains reduced and the proportion of intake air through the bypass passage of the entire intake air amount is large. As a result, if a difference between intake air amount calculated in feedback control and intake air amount calculated in open loop control is large, there is a possibility that a transition from an idling state where engine speed is feedback-controlled to a non-idling state where engine speed is open-loop controlled will not be carried out smoothly, even if the rate reduction in intake air amount is gradual at the time of the transition.